1. Field of the Invention
The present invention is related to the field of geophysical exploration. More specifically, the present invention is related to methods for reducing the effects of noise on the quality of seismic surveys.
2. Description of the Related Art
Seismic surveys are used to determine the presence of subsurface structures in earth formations which may contain useful materials such as petroleum. Seismic survey methods known in the art include positioning a plurality of seismic sensors, which can be geophones, at spaced apart locations in a predetermined pattern on the surface of the earth. The sensors are typically connected to a recording system. Another type of seismic survey includes positioning seismic sensors in a predetermined pattern on the water bottom in a shallow marine seismic survey. Still another type of seismic exploration includes positioning seismic sensors near the water surface in a marine seismic survey. The sensors in marine surveys can be hydrophones.
An acoustic energy source, which can include sources such as dynamite, vibrators or air guns is then activated to impart acoustic energy into the earth. The source is typically located at a predetermined position in the vicinity of the seismic sensors. The acoustic energy from the source radiates outwardly into the earth. Some of the acoustic energy travelling through the earth can be partially reflected back towards the earth's surface by subsurface acoustic impedance boundaries, called reflectors, which may exist within the earth. The reflected acoustic energy can be detected by the sensors. The sensors generate electrical signals proportional to the amplitude of the acoustic energy. The electrical signals from each sensor are typically recorded, with respect to time, in the recording system.
Various forms of images corresponding to structures within the earth can be generated by presenting the recordings of the electrical signals on a 2-dimensional graph. The graph includes "traces", each trace representing an individual seismic sensor signal recording or combinations of signal recordings from a plurality of sensors, positioned on the graph corresponding to the location at which each seismic sensor was positioned during the recording of the survey. Such a graph is referred to as a record section.
The signals generated by the sensors are susceptible to interference by various sources of noise. Noise in the signals reduces the clarity of the images of the subsurface structures within the earth. One source of noise is called "ground roll" Shallow marine surveys can have an equivalent source of noise called "mud roll". Ground roll and mud roll comprise generally up-and-down motion of the surface of the earth or the sea bottom. The motion propagates substantially along the surface of the earth or sea bottom, where the up-and-down motion can be imparted to the geophones, causing signals to be generated which are unrelated to reflection of seismic energy from within the earth. Other sources of noise can include scattered reflections, multiple reflections, wind noise and electrical noise impressed onto the seismic recording system.
Various methods are known in the art for processing the traces to reduce the effects of noise on the seismic signals. One method known in the art is called (f-k) transform filtering. (f-k) transform filtering includes performing a 2-dimensional Fourier transform on a plurality of the traces in order to convert at least a portion of the record section, which is initially a graph with respect to time and position, into a graph with respect to frequency and wavenumber. Noise can be removed from the (f-k) transformed record section by applying a so-called "velocity filter" to the transformed graph. Noise, which frequently exhibits an apparent velocity unrepresentative of the velocity of acoustic energy travelling through earth formations, can be substantially reduced by using velocity filters. A drawback to (f-k) transform filtering is that in order for the 2-dimensional Fourier transform to be precise, the recorded traces must represent seismic sensors which are uniformly spaced apart from each other. It is extremely difficult in practice to achieve truly uniform spacing between sensors. Another drawback to (f-k) transform filtering is that seismic events having a higher spatial frequency than the spacing between seismic sensors will cause error in the transformed traces, the error referred to as "spatial aliasing".
It is also known in the art to reduce noise effects in seismic signals by "slant stacking". Traces in the record section are summed or "stacked" after applying a time offset to each trace which depends on that trace's equivalent distance between the sensor and the seismic energy source. The time offset is referred to as the apparent "moveout", or change in a time of recording of a correlative part of a noise event present on each trace. Slant stacking has the disadvantage of being difficult and expensive to perform. Slant stacking is also susceptible to error caused by dispersion of the noise source. Dispersion is a characteristic of some noise sources which includes the apparent velocity of various components of the noise being dependent on the frequency of those components.
Still another method known in the art for removing noise sources is called median filtering. Median filtering is described, for example in "Vertical Seismic Profiling", Bob A. Hardage, Handbook of Geophysical Exploration Volume 14A, Geophysical Press Limited, London, 1985 (pp. 172-186). Median filtering is used because it can be particularly effective in reducing effects of short duration noise events, called "spikes", from seismic signals. A particular drawback to median filters, however, is that they are non-linear. Median filters applied to seismic signals in different stages of signal processing can have different effects on the final result. Median filters are also susceptible to generation of artifacts, a particular one being called "whiskers", which are described in the Hardage reference, supra, on p. 186.
Accordingly, it is an object of the present invention to provide a method of identifying and reducing noise in seismic signals which is not dependent on uniformity of spacing of the seismic sensors used to record the seismic signals.
It is a further object of the present invention to provide a method for identifying and reducing noise in seismic signals which is not susceptible to spatial aliasing.
It is yet another object of the present invention to provide a method of identifying and reducing noise in seismic signals which is not susceptible to generation of artifacts.
It is still a further object of the present invention to provide a method of identifying and reducing noise in seismic signals which is not substantially affected by dispersive characteristics of the noise.